The advent of heat-recoverable, thermoplastic materials generated a new industry producing articles of such materials for conduit protection. Conduits, such as electrical cabling, pipes and fluid passageways generally require protection against adverse environmental elements (water, corrosive chemicals, etc.). These considerations have led to the development of a host of heat-recoverable, thermoplastic closures and sleeves.
A common characteristic of heat-recoverable closure sleeves is that they have been oriented or stretched in order to induce the shape-memory shrinking which enable the closures to conform to the specific contours and dimensions of the conduit surface. The construction and function of these sheets permit the field worker to wrap a sheet around a conduit at any portion therealong, shrinking the covering sheet to seal the conduit against environmental elements by application of heat. The single, most important feature of most closures is this establishment of a reliable environmental seal. In the case of wraparound sleeves, structures incorporate button-type edge joints, rail and channel edge joints, etc., in order to achieve the intended sealing function. Oftentimes, however, during the course of heat application, the junction of the two edges or the overlap of the wraparound sheet separate as a result of the hoop stress induced during the shrinking process. The hoop stress overcomes the sealing or joining mechanism employed to secure the overlapping edges of the closure sleeve. When such separation occurs, the purpose of the sleeve is defeated, virtually in its entirety.
A further consideration relating to such sleeves is ease of installation. Often, in field installation, two persons are required to effect application of a closure sleeve. One applies the heat necessary to induce shrinking while the other holds the sleeve in position. Alternatively, adjunctive equipment may be substituted for the holder but this delays installation where the installer must first dedicate time and effort to apply such equipment.
Lap shear, for the purpose of this application, will be understood as a measurement of force required to separate a joint formed by the overlapping of two edges of a wraparound sleeve.
Hoop stress is defined as a measurement of the tangential force generated during recovery of a shape-memory material. More particularly, in this case, it represents the shrinking force in the direction of orientation induced upon application of heat at recovery temperature of the polymeric material employed in the heat-recoverable sheet component of the sleeve contemplated by this invention.